This routine applies the Porter Stemming Algorithm to its parameters, returning the stemmed words. It is derived from the C program "stemmer.c" as found in freewais and elsewhere, which contains these notes:

I have re-interpreted areas that use Frakes and Cox's "WordSize" function. My version may misbehave on short words starting with "y", but I can't think of any examples.

The step numbers correspond to Frakes and Cox, and are probably in Porter's article (which I've not seen). Porter's algorithm still has rough spots (e.g current/currency, -ings words), which I've not attempted to cure, although I have added support for the British -ise suffix.